In many advanced telecommunication systems, high performance DC/DC power converters are used to supply conditioned power to electronic cards. Some of the basic requirements for this power conversion are:
1. high power density PA0 2. high efficiency PA0 3. low EMI (both conducted and radiated); and PA0 4. constant operating frequency. PA0 (1) full-resonant DC/DC converters; PA0 (2) quasi-resonant DC/DC converters; and PA0 (3) multi-resonant DC/DC converters PA0 1. Due to mismatching in the values and voltage ratings of the commercially available high frequency capacitors and the actual value and voltage of the parallel branch capacitor, a number of capacitors are normally required to overcome this mismatching and to meet the design requirements. This results in a physically large size parallel resonant branch. PA0 2. Since the parallel branch is connected between the primary of the PA0 2. Since the parallel branch is connected between the primary of the transformer and the series resonant branch, the power transfer capability of the converter decreases at increasing operating frequencies due to the leakage inductance of the transformer. This results in lower power density of the converter. PA0 3. Since the capacitor of the parallel branch is connected between the series inductor and primary winding of the transformer, the three magnetic components (series inductor, parallel inductor and transformer) cannot be integrated into a single magnetic structure.
In order to achieve high power densities, there is a trend to operate power supplies at higher switching (operating) frequencies. As the switching frequencies increase, the switching losses associated with the turn-on and turn-off of the devices in the power supplies also increase. In switch mode power supplies, these losses are so significant that the operation of the power supplies at very high frequencies are prohibitive due to low conversion efficiencies. However, in resonant mode power supplies, the switching losses are low, which allow operation of resonant converters at very high frequencies. The resonant mode DC/DC converter can be classified into the following categories:
A number of U.S. patents have been issued for resonant mode DC/DC converters, e.g. U.S. Pat. Nos. 4,814,962 issued Mar. 21, 1989 (Magalhaes et al); U.S. Pat. No. 4,679,129 issued Jul. 7, 1987 (Sakakibara et al); U.S. Pat. No. 4,355,243 issued Oct. 19, 1982 (Tellert); U.S. Pat. No. 4,935,857 issued Jun. 19, 1990 (Nguyen et al); U.S. Pat. No. 4,833,584 issued May 23, 1989 (Divan) on full-resonant DC/DC converters; U.S. Pat. Nos. 4,720,667 and 4,720,668 issued Jan. 19, 1988 (Lee et al) on quasi-resonant DC/DC converters; and U.S. Pat. Nos. 4,841,220 issued Jun. 20, 1989 and U.S. Pat. No. 4,857,822 issued Aug. 15, 1989 (both to Tabisz et al) on multi-resonant DC/DC converters. The converter topologies described in the above-referenced patents have switching losses reduced to near zero, however, their output voltage is controlled and/or changed by varying the operating (also called switching) frequency. These converters are, therefore, unsuitable for applications where system synchronization is required, i.e. in a telecommunications environment.
Applicant's U.S. Pat. Nos. 5,208,738 issued May 4, 1993, U.S. Pat. No. 5,157,593 issued Oct. 20, 1992 and U.S. Pat. No. 5,159,541 issued Oct. 27, 1992 teach constant frequency resonant DC/DC converters. These converters exhibit near zero switching losses and are suitable for very high frequency operation. From a practical point of view, however, these converter topologies have the following limitations: